Change remaining-time-display.js so that the time value announced by a screen reader is not negative

[OwenEdwards]

Description

As mentioned in #5157, the convention to display the remaining time in a video as a negative number is purely a visual convention; for screen reader users, where the remaining time has a hidden label, the actual amount of remaining time is positive, not negative.

So the remaining-time-display.js component should either add the negative sign in a way that is decorative and not announced by screen readers, or the visual (negative) number should be aria-hidden from screen readers and a hidden positive value should be added.

[smbea]

@gkatsev Hello, this is my approach for fixing the issue. I made it so the '-' is added on every time remaining update in a purely decorative way. I was wondering if this fits the what you had in mind for the solution or it should be a more efficient one.

You can see the screenshots of the solution working:

And here are the changes made: